Name a metric, any metric, for measuring audience attention, and there is (a) a reason why it's useful; (b) a reason why it's worthless; and (c) a way for digital media companies to corrupt it.

Page views (e.g.: clicks) used to be the most common currency of online attention, only to be replaced by unique visitors (e.g.: readers). But in the viral age, "readers" doesn't mean what it used to mean. If you were a newspaper in the 1980s, readers = subscribers who receive your bundle of paper each morning. In a bookmarked-site world, readers meant Web visitors who drop by a few days a month at most. But in the viral age, readers can mean 1 million Facebook users who see some sensational headline, clicked it, and scurried away, having no recollection of what URL hosted the article, and never visit the site again.